Alleviation of lead stress in sunflower (Helianthus annuus L.) by growth-promoting rhizobacterium and neem cake

Lead causes toxicity and affects plant growth by disturbing photosynthesis and the functions of enzymes. Its removal from the soil is a challenging issue. The present study investigates the efficacy of neem cake as soil organic amendment and inoculation of a lead-tolerant plant growth-promoting rhizobacterium (PGPR), Pseudomonas aeruginosa, to decrease the effects of lead toxicity in sunflower, when cultivated with and without neem cake amended soil. The experiment was performed in earthen pots (10 cm diam) containing 1 kg sandy loam soil irrigated with 0.2 and 0.5 mM lead. P. aeruginosa used in this experiment has a lead remediation capacity of 99% when subjected to 0.6 mM lead. The combination of organic amendment with PGPR improved the growth and physiological performance of the stressed sunflower plant. The plants treated with PGPR in amended soil showed healthier roots and shoots and demonstrated better scavenging of free radicals, with improved H2O2 scavenging and peroxidase (POD) activities. Neem cake and P. aeruginosa separately and combined significantly enhanced sugar contents and POD activity while lowering stress-induced higher levels of phenols and ascorbic acid. The combined application of neem cake and P. aeruginosa could be a promising solution for growing sunflower plants in lead-contaminated soil.